SQL Server Lock Escalation

on February 9, 2013

About lock escalation

When more than 5,000 locks are taken out on the rows or pages of a particular table within a single T-SQL statement, lock escalation is triggered. During this process, the intent lock at the higher level is converted to a full lock — assuming this is possible and not prevented by other locks that may be already acquired — and then the locks at the more granular levels can be released, freeing up the resources needed to manage them.

When a lock is taken out on a row or page, intent locks are taken out on the items higher up in the lock hierarchy — in particular, on the HoBTs and tables related to the locked row/page. In addition to providing a shortcut to determining whether something might be locking part of the table, these intent locks provide escalation points if the overhead of maintaining the locks becomes too high.

Escalation is to either the HoBT (for partitioned tables) or to the table itself (which is more typical). A page lock is not considered an escalation point — probably because by the time 5,000 locks are taken out, quite a large number of pages are locked, and a full table lock is a sensible solution to be able to reduce the number of locks.

If escalation can’t occur, the more granular locks can’t be released, and everything continues as before, with locks being taken out at the more granular points. This is typically because of other activity occurring in the affected table. Escalation will be attempted each time another 1,250 locks are acquired.

Preventing lock escalation

Lock escalation can be prevented by setting a table option to disallow it, or by forcing queries to take out table locks to start with. Ideally, you should let the system escalate locks as required, and only consider this kind of action when the number of escalations (monitored through Lock:Escalation events) becomes significantly higher than expected (compared to a benchmark of your system in a healthy state). You can also use trace flags (1211 and 1224) to disable lock escalation.

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